U.S. patent application Ser. No. 405,258, now U.S. Pat. No. 5,089,363, filed Sep. 11, 1989 in the name of Rimai et al, and U.S. Pat. No. 5,023,038, issued Jun 11, 1991 in the name of Aslam et al disclose a method and apparatus for fixing a multicolor toner image carried on a heat softenable outside layer of a receiving sheet. The receiving sheet is passed across a preheating plate to raise the temperature of the thermoplastic layer to or above its softening point. It is fed into a pressure nip created by a pressure roller and a belt or web backed by a heated roller. The belt or web is of a hard ferrotyping material such as stainless steel, nickel or the like. Relatively high pressure is applied between the belt and pressure roller to embed much or all of the toner image in the thermoplastic layer fixing the image. Some of the toner may be not entirely embedded but may be fused on the top of the layer, but with much of it embedded, the hard ferrotyping belt provides a photographic quality with an absence of relief and a high gloss. The image and heat softenable layer are retained in contact with the belt as it moves away from the pressure nip. The belt and receiving sheet are allowed to cool until the heat softenable layer is below its glass transition temperature. At this point it can be separated without offset. All this is accomplished without the use of offset-preventing liquids which would reduce the photographic quality of the image.
In the methods shown in these applications, the back of the receiving sheet is contacted by a heated plate just prior to the receiving sheet entering the pressure nip. The heated plate preheats the receiver up to or approaching the glass transition temperature of the heat softenable layer. This preheating permits the moisture in the receiving sheet to gradually escape while not constrained by a pressure nip and also allows the nip itself to be quite narrow as is generally required in higher pressure systems.
This approach to preheating, of course, requires a separate element, and space must be provided for it. To be most effective, it must be positioned as close as possible to the nip.